Reason why l can be getting horriblegas mileage on my Chevy s10

you could take it to an AutoZone or some other parts store that offers free testing of the computer. that will tell you if you have any error codes or anything. If you used to get better gas mileage and now it is suddenly horrible, it could be your oxygen sensor. when that goes bad, your computer is reading wrong signals and adjusting your fuel mix incorrectly. i've had to change them in many cars.